1. Field of the Invention
The present invention relates to a differential limiting control apparatus provided in a center differential of a four wheel drive vehicle for performing a differential limiting control between a front drive shaft on the front wheel side and a rear drive shaft on the rear wheel side.
2. Discussion of Related Arts
In general, many of power transmission controls such as front-rear wheels power distribution controls or front-rear/left-right wheels differential limiting controls, variably control the clutch engagement force by a multiple disc clutch and the like. With respect to the differential limiting control in which the clutch engagement force is variably controlled, the vehicle driving performance is largely dependant on the control characteristics of the differential limiting control. Further, the vehicle driving performance varies according to tires or road surface conditions with the same control logics and control constants. Further, the required vehicle driving performance differs with an individual driver or with driving conditions.
Hence, Japanese Patent Application Laid-open Toku-Kai-Hei 8-132914 discloses a technology of a front-rear wheels torque distribution control apparatus in which a driver establishes a differential limiting torque by the manual operation to obtain a desired torque distribution condition.
In order to realize the driving performance of the vehicle as a driver desires, however, it is necessary to variably control the differential limiting torque according to a variety of driving conditions or road surface conditions. Therefore, it is difficult to adjust the differential limiting torque to a target value and to realize the most suitable driving performance of the vehicle merely by establishing the differential limiting torque directly by the manual operation of the driver, as proposed in Toku-Kai-Hei 8-132914. Further, in case where the driver manually operates the differential limiting torque control apparatus and drives the vehicle, for example, in a released condition of the differential limiting clutch as the driver intends, in an extreme case, a spin may occur to the vehicle on a road surface with low friction coefficient. Reversely, in case where the driver manually operates the differential limiting torque to drive the vehicle in an engaged condition of the differential limiting clutch for a long time, sometimes wrong loads such as an internal circulation torque occurs to the powertrain of the vehicle. Further, such wrong loads may exacerbate fuel economy.